Any time someone tries a new method of execution, activists pounce on it and take it to court, making any new method expensive because of court costs and tied up in years of cases. The only methods of execution that can't be filibustered in this way are methods where courts have specifically ruled in the past that the method is okay. That's why execution is limited to specific methods, and why we're not going to use carbon monoxide or nitrogen.

Personally, I'd prefer that if the extra suffering caused by a method of execution is less than the suffering caused by, for instance, a week in jail, we should ignore the suffering and permit use of the method.